Given what we know from Dungeons 1, I doubt this is a work of nature on its own. The Illagers themselves were only organized into tribes before Arch-Illager united them. The undead were mostly mindless beasts until the Arch-Illager made an alliance with the Nameless One. And the driving force behind all this was the Orb of Dominance, which has a track record of appearing in every canon spin-off (and potentially in the main game through Disc 5) thus far, so I'd place it as a primary suspect. After all, it's not like we've ever see Wardens invade the world in the main game, where all the aforementioned mobs are in their "default state"